Filled vias are utilized in circuit manufacturing. As an example, a circuit may be formed on each side of a substantially flat substrate. To electrically connect elements on opposing sides of the substrate, a hole or via may be formed in the substrate, which is filled with a conductive material or fill. The electrical elements then utilize the conductive material to form an electrical path through the substrate. The hole filled with conductive material is an example of a filled via.
One common method of forming a filled via is to insert a paste made substantially with metal and glass. The paste is heated to a high temperature and, as it cools within the via, it forms a mechanical connection with the walls of the via. Up to approximately 80-85% of the paste may be metal and at least 10% of the paste may be glass. The glass is important because without the glass, or a comparable substance, the metal would be unable to form an adequate mechanical connection with the walls of the via. Also, other conductive metals beyond gold may be utilized in the paste, but gold is recognized as having more favorable properties as compared with other available metals. Tungsten and Copper Tungsten are examples of other compatible metals, for this process, although each still requires a significant percentage of glass or comparable substance.
FIG. 1 is an illustration of a cross-sectional view of a metal-glass paste composite filled via 10 as is known in a first exemplary embodiment of the prior art. A substrate 12 is shown with a hole formed therein, bordered by a wall 14. Within the wall 14, a first paste 16 is utilized to fill the hole, heated and cooled as previously described. The first paste 16 is usually applied and fired at atmosphere. Due to differences in the coefficient of thermal expansion of the first paste 16 and the substrate 12, gaps or crevices may develop between the wall 14 and the first paste 16 as it cools. A second paste 18 is thereafter applied and fired in a second process of heating and cooling. And a third paste 20 is thereafter applied and fired in a third process of heating and cooling. Each of the firing processes may take place in atmosphere or in special environments. The second and third pastes 18, 20 are applied to seal the first paste 16 and prevent foreign substances from entering any gaps in the first paste 16. However, gaps or voids in any of the layers may cause blisters in the pastes 16, 18, 20 during subsequent processing. Since these circuits may be used in high-performance military or commercial applications, blisters compromise the integrity of the filled via 10, increase cost and decrease reliability.
While described as ‘pastes’, it should be noted that, after cooling, each of the pastes has the approximate rigidity of metal. Also, as glass is not a good conductor of heat or electricity, a metal and glass paste is not as good a conductor of heat or electricity as a pure metal system would be.
FIG. 2 is an illustration of a cross-sectional view of an electroplate filled via 10A as is known in a second exemplary embodiment of the prior art. A substrate 12A is shown with a hole formed therein, bordered by a wall 14A. A conductive film 16A is applied to one side of the substrate. The substrate is then electroplated with a material 18A and the via is filled to form metal “pillars” 20A in the via. These vias are not well bonded to the substrate 12A and suffer blistering and reliability issues similar to the metal and glass composite filled vias.
A via filling process that requires fewer process steps, provides a hermetic bond to the substrate, and/or allows for greater thermal and electrical conductivity would be desirable.